The source does not define C explicitly. For the positively oriented contour enclosing the only pole z=i/3, as intended by its official key, the residue is [(i/3)³−9]/3=−3−i/81. Multiplication by 2πi gives 2π/81−6πi, A. A contour excluding the pole would give zero, so enclosure/orientation is a necessary implicit assumption.
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